Answer
$$1460 \ km .$$
Work Step by Step
Since we have a right triangle with legs a, b and the hypotenuse c, then by the Pythagorean theorem, we have $$c^2=a^2+b^2.$$
Since $c=2460 km$ and $a=1980 km$, then $a$ is given by
$$ b=\sqrt{c^2-a^2}=\sqrt{2460^2-1980^2}=1460 \ km .$$
